1. Field of the Invention
This invention relates to a deposited film formation method, particularly to a formation method of Al or Al-Si deposited film which can be preferably applied to wiring of a semiconductor integrated circuit device, etc.
2. Related Background Art
In the prior art, in electronic devices or integrated circuits using semiconductors, for electrodes and wiring, aluminum (Al) or Al-Si, etc. have been primarily used. Here, Al has many advantages in that it is inexpensive and high in electroconductivity, that it can be also be internally chemically protected because a dense oxidized film can be formed on the surface, and that it has good adhesion to Si, etc.
The integration degree of the integrated circuit such as LSI, etc. is increased, and fine formation of wiring and multi-layer wiring, etc. have been particularly required in recent years. For this reason, there is an increasing heavy demand to meet those requirements which have not been found to date for Al or Al-Si wiring in the prior art. With finer dimensional formation by increased integration degree of the integrated circuit, the surface of LSI, etc. is subject to excessive unevenness due to oxidation, diffusion, thin film deposition, etching, etc.
Electrodes or wiring metal must be deposited on the surface with a stepped difference, or deposited in a via-hole which is fine in diameter and deep. In 4 Mbit or 16 Mbit DRAM (dynamic RAM), etc., the aspect ratio (via-hole depth/via-hole diameter) of via-hole in which a metal such as, Al-Si, etc. is deposited is 1.0 or more, and the via-hole diameter itself also becomes 1 .mu.m or less. Therefore, even for a via-hole with large aspect ratio, a technique which can surely deposit a metal such as Al, Al-Si, etc. is required.
Particularly, for performing sure electrical connection to the device under insulating film such as SiO.sub.2, etc., rather than film formation, Al or Al-Si is required to be deposited so as to be embedded in only the via-hole of the device. For such purpose, a method of depositing Al or Al-Si on Si or metal surface and not depositing it on an insulating film such as SiO.sub.2, etc. is required.
Such selective deposition or selective growth cannot be realized by the sputtering method which has been used in the prior art. Since the sputtering method is a physical deposition method based on flying of the particles sputtered from the target in vacuum, the film thickness at the stepped portion or the side wall of insulating film is liable to become extremely thin, even resulting in wire breaking in an extreme case. Additionally, nonuniformity of the film thickness and wire breaking will markedly lower reliability and yield of LSI.
Accordingly, there has been developed the bias sputtering method in which a bias is applied to a substrate and deposition is performed so as to only embed Al or Al-Si in the via-hole by utilizing the sputter etching action and the deposition action on the substrate surface. However, since a bias voltage of some 100 V or higher is applied to the substrate, deleterious influence on the device may sometimes occur because of charged particle damaging such as change in threshold value of MOS-FET, etc. Also, because of coexistence of both etching action and deposition, there is the problem that the deposition rate cannot be essentially improved.
In order to solve the problems as described above, various types of CVD (Chemical Vapor Deposition) methods have been proposed. In these methods, chemical reaction of the starting gas is utilized in some form. In plasma CVD or optical CVD, decomposition of the starting gas occurs in gas phase, and the active species formed there further reacts on the substrate to form a film. In these CVD methods, surface coverage over unevenness on the substrate surface is good. However, since an organometallic compound having alkyl group bonded to Al atom is generally used as the starting gas, there is the problem that the carbon atoms contained in the starting gas molecule are incorporated into the film. Also, particularly in plasma CVD, the problem remained that damage by charged particles (so called plasma damage) may occur at some time as in the case of the sputtering method.
In contrast, the thermal CVD method has good coverage over unevenness such as stepped difference of the surface, because the film grows according to the surface reaction on the surface of substrate. Also, deposition within via-hole can be expected to readily occur. Further, due to good coverage, wire breaking at the stepped difference can also be avoided.
For example, as the method for forming Al film by the thermal CVD, there may be employed the method in which an organic aluminum is transported onto a heated substrate and the gas molecules are pyrolyzed on the substrate to form a film. For example, in an example shown in Journal of Electrochemical Society, Vol. 131, p. 2175 (1984), by use of triisobutyl aluminum (i-C.sub.4 H.sub.9).sub.3 Al (TIBA) as organic aluminum gas, film formation is effected at a film formation temperature of 260.degree. C. and a reaction tube pressure of 0.5 torr to form a film of 3.4 .mu..OMEGA..multidot.cm.
However, when TIBA is employed, no continuous film can be obtained unless such pre-treatments as flowing of TiCl.sub.4 before film formation, activation of the substrate surface, formation of nucleus, etc. are applied. Also, including the case using TiCl.sub.4, generally there is involved the problem that surface selectivity is inferior when TIBA is employed.
Japanese Laid-open Patent Application No. Sho-63-33569 describes a method of forming a film by using no TiCl.sub.4, but using in place thereof organic aluminum and heating it in the vicinity of the substrate. According to this method, Al can be deposited selectively only on the metal or semiconductor surface from which the naturally oxidized film has been removed.
However, in this case, it is clearly described that the step of removing the naturally oxidized film on the substrate surface is necessary before introduction of TIBA. Also, it is described that, since TIBA can be used alone, no carrier gas is required to be used, but Ar gas may be also used as the carrier gas. However, the reaction of TIBA with another gas (e.g. H.sub.2) is not contemplated at all, and there is no description of use of H.sub.2 as the carrier gas. Also, in addition to TIBA, trimethyl aluminum (TMA) and triethyl aluminum (TEA) are mentioned, but there is no specific description of other organometallic compounds. This is because, since the chemical properties of the organometallic compounds generally vary greatly if the organic substituent attached to the metal element varies little, it is necessary to investigate individually to determine what organometallic compound should be used. In this method, not only is there the inconvenience that the naturally oxidized film must be removed, but also there is the problem that surface smoothness cannot be obtained.
In the pre-text of the 2nd Symposium of Electrochemical Society, Branch of Japan (July 7, 1989), on page 75, there is a description of film formation of Al according to the double wall CVD method. In this method, TIBA is used and the device is designed so that the gas temperature can be made higher than the substrate temperature. This method may be also regarded as a modification of the above-mentioned Japanese Laid-open Patent Application No. Sho-63-33569. Also in this method, Al can be selectively grown only on a metal or semiconductor. However, not only the difference between the gas temperature and the substrate surface temperature can be controlled with difficulty, but there is also the problem that the bomb and the pipeline must be heated. Moreover, according to this method, there are involved such problems that no uniform continuous film can be formed, that the flatness of the film is poor, that selectivity of Al selective growth cannot be maintained for so long a time, etc., unless the film is made thick to some extent.
Also, since TIBA has small vapor pressure at room temperature as 0.1 torr, it has the problem that it is transported in a large quantity with difficulty. For this reason, it has been the practice to heat the bomb for organometallic compound to 45.degree. to 50.degree. C., but there is the problem that not only the vessel for organometallic compound, but also the pipeline to the reaction vessel must be also heated.
Trimethyl aluminum (TMA) has a vapor pressure of approximately 10 torr at room temperature, and it is possible to transport TMA to a reaction vessel efficiently by flowing a gas such as H.sub.2, etc. into the TMA starting material solution.
J. Electrochem. Soc. 135 (2) (1988) 455 discloses that deposition of Al is possible by use of TMA according to the plasma CVD method or the magnetron plasma CVD method.
However, in the magnetron CVD method, since even C--H bond of alkyl group is decomposed, there is the great problem that several % to several 10% of carbon atoms are incorporated in the deposited film. Also, there is the problem of occurrence of charged particle damaging by plasma.
Since the magnetron plasma CVD method is based on decomposition of TMA in the gas phase, it can be deposited on the substrate surface, whether it may be Si or SiO.sub.2, whereby selective Al deposition is essentially impossible.
According to Japanese Patent Application No. Sho-60-211149, when TMA is used, Al deposition without incorporation of carbon is possible according to the reaction with heat or heat and UV-ray irradiation by use of excited and decomposed molecules, by exciting and decomposing TMA molecules with plasma in the gas phase apart from the wafer.
Similarly, Extended Abs. of 20th Solid State Devices and Materials (1988) p. 573 by the present inventors, there is shown deposition of an Al film without incorporation of carbon at a deposition rate of 10 to 20 .ANG./min. on Si or a thermally oxidized SiO.sub.2 wafer heated to 260.degree. C. to 350.degree. C. by use of TMA and by exciting TMA with an H.sub.2 atmosphere high frequency (73.56 MHz) plasma.
According to the method shown in Japanese Patent Application No. Sho-60-211149 and Extended Abstract of 20th Solid State Devices and Materials (1988) p. 573, although deposition example of Al thin film is shown, there is no reference to the method of depositing Al selectively only on the Si exposed portion on the Si wafer subjected to patterning of SiO.sub.2. In Extended Abstract of 20th Solid State Devices and Materials (1988) p. 573, it is shown that Al is deposited at the similar extent of deposition rate on the thermally oxidized SiO.sub.2 wafer and Si wafer.